Electric motors that utilize permanent magnets and magnet coils are known, having been disclosed in JP2001-298982A, for example.
In the prior art electric motors, motor control is carried out using an on/off signal from a digital magnetic sensor. Specifically, the timing for reversing the polarity of the voltage applied to the magnet coil is determined using the on/off signal from the digital magnetic sensor.
Magnetic sensors having analog output are also available. However, where an analog magnetic sensor is used to control a motor, considerable error in sensor output can occur due to manufacturing errors of various kinds among motors, making it impossible in some instances to carry out motor control satisfactorily. Examples of manufacturing errors among motors that can have an effect on output of an analog magnetic sensor are error in the installation location of the magnetic sensor; error in the location of the N pole/S pole boundary due to magnetization error of the permanent magnet; and error in mounting location of elements inside the magnetic sensor. However, to date there have yet to be devised satisfactory technology for achieving accurate motor control using analog magnetic sensors, while taking such errors into consideration. This problem is not limited to cases where analog magnetic sensors are used; the problem is encountered also in cases where digital magnetic sensors having multivalue analog output are used.
Moreover, in conventional control employing the on/off signal from a digital magnetic sensor, since the effective voltage applied to the electric motor has rectangular shape, efficiency declines at the edge of the rectangular voltage waveform, and this creates the problem of considerable noise and vibration.
Incidentally, PWM control circuits for use in motor control typically produce PWM signals by comparing a reference triangular signal and a sinusoidal signal which is generated from a motor sensor output. In response to variation of load change for the motor, the level of the triangular signal is modified so as to make the motor to properly operate according to the load. However, conventional PWM control circuits have fairly complicated circuitry structure in order to generate the PWM signals in response to a motor load change. This problem is not limited to the PWM control circuits for motor control, but in common with various PWM control circuits for other purposes.